Thermionic amplifier



May 13, 1941. H O HE 2,241,925

THERMIONIC AMPLIFIER Filed April 5, 1939 2 Sheets-Shet l M/ VENTOR 24.ROCHE y 1941- I A. H. ROCHE 2,241,925

THEBMIONIC AMPLIFIER Filed April 5, 1939 .2 Sheets-Sheet 2 Fig. 3.

GAIN //V 06 Patented May 13, 1941 to International,

Standard, Electric Cl i 'nfirae tion, New York, N. Y.

Application April, 5, 1939, Scrial No. 266,129,

In Great Britain April 121, 1938 Claims.

This invention relates to thermionic amplifiers and more particularly tosuch amplifiers in which a feedback circuit is provided for feeding backwaves including those of the range of; transmitted frequencies from theoutput to the input of the amplifier to reduce the gain of the amplifierbelow the value it would have without feedback, in order to reduceunwanted modulation and/or non-linear effects and to render the gainstability greater than it would be without feedback.

The invention consists more particularly in improvements in ormodifications of feedback amplifiers in which the feedback voltageapplied to the input is obtained froma tapping 'ona reactivepotentiometer which is shunted across the feedback path and theimpedance of which is large compared with the impedance presented to itby the feedback path. Such an arrangement provides a, simple means ofadjusting both the gain and equalization ofthe amplifier.

According to the present invention in an amplifier as described in thepreceding paragraph, one or more networks is or are also incorporated ain the feedback path, which network or networks is or are sodesignedthat the phase and gain characteristics of the amplifier outsidethe operating range are modified in such manner that instability isprevented whilst leaving the gain substantially unaffected in theoperating range. Preferably the network or networks is or are shuntedacross the feedback path in series with an impedance which is lowcompared with the impedance of the reactive potentiometer. In analternative arrangement in which the potentiometer comprises a reactiveportion and a resistive portion the latter being included in the gridcircuit of a, valve of the amplifier and in which a low impedance isshunted across the potentiometer, the network or networks is or areinserted between the junction of the resistive portion and the lowimpedance and the cathode of the valve.

In the accompanying drawings, l and 2 show two embodiments of theinvention and Fig. 3 is a curve used in the explanation of the inventionin t -e following detailed description.

Referring to Fig. 1 this shows an amplifier of the kind described inBritish Patent No. 472,256 but only sufiicient of the circuit is shownfor a proper understanding of the present invention.

The output bridge consists of the resistance RH shunted by condenserCit, the resistance R56, the resistance R55 and condenser C12 in seriesand the plate cathode impedance of the valve'va. This is. a. balancedahidac and the-c ndenser Cl8 is made variable for adjustment of the outputimpedance, athigh frequenciesandthe resistance Rl5 is also madeslightly. variable to give a measure of output impedance adjustment overthe whole frequency range, as, described in the above-mentioned patent.

The feedback voltage is. taken across thev di-.-. agonals of the outputbridge in thenormalmanner. Thus a lead is taken, fromthe, junction ofRES and RH and isconnected through. theimpedance R26, L8. and (32;!v to,the feedback resistance R22. shunted directly across. the output bridgefeedback diagonals is the circuit con--. sisting ofR-ZS in. series with.R24 and L9-= in parallelin series with R25 and Cl9-in parallel. Theelements CH9 and B25 in para11el: are insertedto give enhanced stabilityat the lower end of the frequency spectrum, as described. in BritishPatent. 499,315 complete. accepted January- 23, 1939, and the elementsR24 and L9 .in parallel serve to correct the upper end of the frequencyspectrum, as described in thesam-e. patent. The. combination of thesevtwo correcting networks forms a simple. band. elimination filter andenables a considerably greater. amount, of. feed. back tobe employed.without fear of oscillation. than would. otherwise. be. possible. The.resistance R23. is a. low impedance shunt whose. value. is smallvcompared. with. the. impedance of. R22 and the equalising network(consistingof R26, L8 and C2! in parallel) in series.

The frequency range covered by a typical amplifier for use in an openwire carrier broadcast channel is 34-425 kilocycles. The stabilitycorrecting networks, comprising elements CH9, R25, R24 and L9, have noeffect on the frequency characteristic in this range. The equalisingnetwork, consisting of the elements R26, Lil and C2! is inserted inorder to correct the gain frequency characteristic of the amplifier forthe losses introduced by the line separating filters which are used toseparate the 3-channel equipment from the broadcast channel. Thetype ofcorrection obtained is shown in Fig. 3 in which the gain in db. of theamplifier is plotted against frequency in kc. It will be seen that overthe range 34 to 42.5 kc. the variation is approximately 3.3 db. with thehighest gain at 34 kc. In order to provide for the manufacturingvariations in the line filters, the equaliser has been made variable byproviding a small adjustment on the condenser C2 i.

The feedback voltage is taken from a tapping on a high impedancepotentiometer shunted across the feedback path.

The high impedance potentionmeter shunted across the feedback pathcomprises the feedback resistance R22 in series with the equalisingnetwork consisting of R26, L8 and C21 in parallel. R23 is a lowimpedance shunt across the diagonals of the output bridge.

In order to understand why it is necessary to insert the stabilitycorrecting networks in series with the low impedance shunt R23, it isnecessary to consider the operation outside the pass range of theamplifier. At the frequencies at which the stability correcting networkstake effect, the impedance of the equalising network R26, L8 and C2l inparallel will be negligible, and we may therefore consider R22 asconnected directly across the feedback path. It has already been statedthat the impedance of R22 is high compared with that of R23, and theeffect, therefore, of shunting R22 across R23 will be almost negligi bleas regards the voltage generated across R23. Any correcting networksinserted in series with R23 will cause a variation in voltage across theresistance R23 at the required frequencies. On the other hand, anynetworks or impedances inserted in series with R22 will have anegligible effect on the feed back voltage because R22 is already highin impedance compared with R23. This is borne out in practice and noincrease in stability is obtained by the insertion of correctingnetworks in series with R22.

An alternative position for the correcting networks is shown in Fig. 2.In this case, the correcting networks are placed between the junction ofR22 and R23 and the cathode lead as shown. In this case, of course, thefeedback voltage is corrected by the'insertion of the stabilisingnetworks because these are in series with R23 as well as R22.

The values of the various components comprising the stabilising andequalising networks to give a curve such as that shown in Fig. 3, may beas follows:

R122 ohms 830 R23 do 29 R24 do.. 1000 R25 do 5000 R26 do 1162 L8mil1ihenries .588 L9 microhenries 11 C I 9 m. f. 2

The design of the amplifier is such that if the equalising element isshorted out, a flat gain over the range 34 to 42.5 kc. of 60 db. isobtained.

What is claimed is:

1. A negative feedback amplifier circuit for amplifying a given range offrequencies, which comprises an amplifier having an input and outputchannel, a feedback channel energized from said output channel, areactive potentiometer of substantially higher impedance than saidfeedback channel bridged across said energized feedback channel so as todraw a current therefrom, a

circuit branch of substantially lower impedance than said potentiometeralso bridged across said energized feedback channel so as to draw asubstantially larger current therefrom, connections from one side ofsaid feedback channel and from an intermediate point of saidpotentiometer respectively to said input channel, and a networkconnected to be traversed by said substantially larger current andproportioned to vary the gain of the circuit only outside said givenrange and in such sense as to increase stability.

2. A negative feedback amplifier circuit according to claim 1, whereinsaid network is serially connected in said lower impedance circuitbranch so as to be traversed solely by said substantially largercurrent.

3. A negative feedback amplifier as claimed in claim 1, in which saidpotentiometer comprises a reactive portion and a resistive portion saidresistive portion being included in the grid circuit of a valve of theamplifier, and said network being inserted between the junction of saidresistive portion and said circuit branch and the cathode of said valve.

4. A negative feedback amplifier as claimed in claim 1, in whichimpedances are connected with the output of said amplifier to form abridge circuit and said feedback channel is energized from the diagonalsof said bridge.

5. A negative feedback amplifier circuit according to claim 1, whereinsaid network is serially connected in one side of said feedback channelso as to be traversed both by said substantially larger current and alsoby said current through said potentiometer.

ALLEMAN HOLLY ROCHE.

